Asphalt based materials are extensively used in a wide variety of applications. For example, asphaltic material is widely employed as a primary ingredient in coating compositions for structures, in sealants, and in waterproofing agents. Asphalt compositions have been used in paving mixtures with considerable advantage for many years. Many manufactured roofing materials, such as roofing shingles, impregnated felts, tars, mastics, and cements, are also based on asphalt and compositions thereof.
In the case of paving asphalts, a typical paving asphalt mixture comprises a mixture of components, principal ingredients of the paving asphalt mixture being an asphalt composition or cement and aggregate or aggregate material. In such mixtures, the ratio of asphalt composition to aggregate material varies, for example, according to aggregate material type and the nature of the asphalt composition. As used herein, the terms "asphalt composition" or "asphalt cement" are understood to refer to any of a variety of organic materials, solid or semi-solid at room temperature, which gradually liquefy when heated, and in which the predominant constituents are naturally occurring bitumens or residues commonly obtained in petroleum, synthetic petroleum, or shale oil refining, or from coal tar, or the like. For example, vacuum tower bottoms produced during the refining of conventional or synthetic petroleum oils is a common residue material useful as asphalt composition. A "paving asphalt composition" or "paving asphalt cement", accordingly, is an asphalt composition or asphalt cement having characteristics which dispose the composition to use as a paving material, as contrasted, for example, with an asphalt composition suited for use as a roofing material. "Roofing asphalts", for example, usually have a higher softening point, and are thus are more resistant to flow from heat on roofs, the higher softening point generally being imparted by the air blowing processes by which they are commonly produced. Paving asphalt mixtures may be formed and applied in a variety of ways, as well understood by those skilled in the art. For example, the paving asphalt composition and the aggregate can be mixed and applied at elevated temperatures at the fluid state of the paving asphalt composition to form the pavement or road surface.
Regardless of the particular manner of formation or application of the particular asphalt mixture, durability is an important matter in the case of most such materials. For instance, in the case of a paving asphalt composition, durability of the road surface is a primary concern. The degree and rate of hardening of the paving asphalt composition or cement during application and while in service (so-called "age hardening"), caused by an increase in viscosity of the asphalt mixture and gradual loss of flexibility, are factors affecting the durability of an applied surface. At least one worker in the art (i.e., Januschke, Industrial Engineering Chemistry Product Research and Development, Vol. 10, 1971, 209-213) has concluded that the reaction of the asphalt composition with atmospheric oxygen is the principal cause of asphalt hardening in pavement. Accordingly, the retardation of hardening by the use of antioxidants has been the object of extensive study. The promulgation of the Strategic Highway Research Program (SHRP) specifications has given further impetus to work on improvement of paving asphalt properties.
Notwithstanding the considerable previous efforts expended to provide asphalt compositions having improved oxidation resistance, or to provide a method of improving asphalt durability, there has remained a continuing need, particularly with paving asphalts, for improved compositions and methods. The invention addresses this need.